In power systems it is known that optimum performance is obtained when the power factor--the ratio of real power to apparent power--is as close to 1.0 as possible. To obtain this optimum power factor, the alternating input current must be in phase and of the same waveshape as the alternating input voltage. Various circuits are known to achieve a power factor of about 0.9 or less. This less than optimal power factor is typically caused by the effect of non-linear rectifier input circuits. For example, in single phase alternating current switching power supplies, also known as power converters, the rectifiers only conduct at the peak AC voltage such that the instantaneous current charging a connected storage capacitor is much larger than the direct current load. The switching power supply draws current from the AC line in short pulses near the peaks of the AC sinusoidal voltage waveshape during the rectifiers forward induction. The resulting input line current distortion is caused by the effect of the non-linear rectifier input circuit. These switching power systems, with less than an ideal power factor, have numerous problems. In particular, line current distortion: (1) reduces the efficiency of the AC power system; (2) requires an increase in line currents to obtain the desired output power; (3) increases line voltage distortion, cross talk and electromagnetic interference; and (4) causes mechanical wear and acoustical noise in the generating equipment.
Currently, there are several methods for optimizing the power factor. This is typically done by means of a circuit between the power supply and the load, wherein the circuit reshapes the input current pulse into a near sinusold waveshape. The two most common methods for achieving this are by employing multi-tap transformers or active power factor corrected preregulators.
The multi-tap transformers are employed with a three-phase power system to minimize and widen the peak rectifier currents. These peak rectifier currents may also be further reshaped by filter inductors. By employing multi-tap transformers, the input current is reshaped into a series of trapezoids that closely resemble the sinusoidal input voltage waveshape. Accordingly, as the number of taps are increased, the number of steps in the waveshape are increased, resulting in a reduction of the total harmonic distortion. This method of power factor correction is typically employed in switching power supplies between the 10 kilowatt to 100 kilowatt range.
Active power factor corrected preregulators can be used on either single phase or multiple phase alternating current inputs. For single phase operation, an active power factor corrected preregulator reshapes the input current into a sinusoid by using a high frequency, pulse width modulated (PWM) power converter. Power converters can employ either a boost, buck, or flyback topology. As is well known, a boost topology or step-up converter produces a higher voltage at the load than the supply voltage. A buck topology or step-down converter produces a lower voltage at the load than the supply voltage and the flyback topology allows for generating a higher or lower voltage at the load as required. Typically, the PWM power converter forces a rectifier within the preregulator to conduct throughout the entire input voltage cycle and regulates the output DC voltage. The preregulator includes a control circuit which consists of: a sensing circuit to determine the input voltage, the input current and the output voltage; and an analog circuit to generate an error signal that is submitted to the pulse width modulator. Depending on the desired configuration of the power converter, different control schemes for the pulse width modulation are voltage mode control, current mode control and voltage feed-forward control. The control circuit employs analog computational methods to determine the duty cycle of the pulse width modulator power converter. For three-phase operation, it is known to employ three single phase active power factor corrected preregulators in a parallel-redundant mode. If one of the preregulators were to fail, the other two preregulators continue to operate to maintain the required output.
Although this existing technology has improved the power factor for power converters, these methods have drawbacks particular to each approach. The multi-tap transformers require the use of bulky transformers and associated analog components. In particular applications it has been found that the size and complexity of these type of power factor correction devices is unacceptable. The active power factor corrected preregulators, while an improvement over the multi-tap transformers, still require dedicated integrated circuits and analog computational circuitry for each phase of operation. As such, if one of the preregulators or one of the phases is rendered inoperative, the phase load becomes unbalanced and causes detrimental line harmonics. Moreover, none of the existing technology provides for easy adaptation to different types of inputs or outputs. It will also be appreciated that these analog type controllers are noise sensitive and difficult to stabilize due to the electrically noisy areas in which they operate. Additionally, digital current PWM converters have a limited frequency range of up to about 30 Khz.
Based upon the foregoing, it is evident that there is a need in the art for a digital controller with a digital pulse width modulator employed in an active power factor preregulator. Furthermore, there is a need for a digital controller with a digital pulse width modulator that can be used with different types of power converters, which is programmable and which is unaffected by electrical noise. There is also a need for a digital controller with a digital pulse width modulator which provides an optimum power factor in any type of power converter.